1. Field of the Invention
The present invention relates generally to a photoprotective composition. More particularly, the present invention relates to a sunscreen composition having optimized polarity, critical wavelength, SPF PFA, photostability, Star Rating, or any combinations thereof. The present invention also relates generally to a method of optimizing photoprotective compositions.
2. Description of the Prior Art
Sunscreen compositions are applied to the skin to protect the skin from the sun's ultraviolet rays that can lead to erythema, a reddening of the skin also known as sunburn. Sunlight or ultraviolet radiation in the UV-B range has a wavelength of 290 nm to 320 nm and is known to be the primary cause of sunburn. Ultraviolet rays at a wavelength of 320 nm to 400 nm, known as UV-A radiation, produces tanning of the skin. However, in the process of doing so, the UV-A rays can damage or harm the skin.
Besides the immediate malady of sunburn, excessive sunlight exposure can lead to skin disorders. For instance, prolonged and constant exposure to the sun may lead to actinic keratoses and carcinomas. Another long-term effect is premature aging of the skin. This condition is characterized by skin that is wrinkled, cracked and has lost its elasticity.
As stated above, sunscreens are typically formulated with the goal of inhibiting skin damage from the sun's rays. The sunscreen composition filters or blocks the harmful UV-A and UV-B rays that can damage and harm the skin. It is believed that sunscreen agents accomplish this by absorbing the UV-A and/or UV-B rays.
Typically, the above-described UV-B filters are combined with the above-described UV-A filters in a solution with other lipophilic or oily ingredients and solvents to form an oil phase. The solvents are used to dissolve the sunscreen actives into the oil phase. Typically, but not necessarily, the oil phase is dispersed with the help of emulsifiers and stabilizers into an aqueous solution composed primarily of water, to make an emulsion, which becomes the final sunscreen composition.
One problem associated with the use of UV filters, and especially those that are rapidly-degrading photoactive compounds, is that they are not photostable and will degrade rapidly and exponentially when exposed to UV radiation. The organic UV-A filters most commonly used in commercial sunscreen compositions are the dibenzoylmethane derivatives, particularly 4-(1,1-dimethylethyl)-4′-methoxydibenzoylmethane (also called avobenzone, sold under the brand name PARSOL 1789). Other dibenzoylmethane derivatives described as UV-A filters are disclosed in U.S. Pat. Nos. 4,489,057; 4,387,089 and 4,562,067, the disclosures of which are hereby incorporated herein by reference. It is also well known that the above described UV-A filters, particularly the dibenzoylmethane derivatives, can suffer in rapid photochemical degradation, when used alone or when combined with the above-described most commercially used UV-B filters. Thus, the efficiency of the sunscreen composition (i.e., SPF, PFA, critical wavelength, Star Rating) containing these photoactive compounds is compromised, unless the photodegradation is controlled by improving the photostability of the system in UVA and/or UVB regions.
By controlling the polarity of the solvent used in a sunscreen composition, the rate of photodecay of the photoactive compounds in the composition can be controlled. By controlling the polarity, greater stability is imparted to the photoactive compounds, thus resulting in a more stable overall composition. The dielectric constant, for example, is a good indicator or measure of polarity in a composition. This is due to the fact that the dielectric constant is a measure of both inherent and inducible dipole moments.
In addition, polar solvents tend to decrease the energy required to excite a pi-bonding electron, and increase the energy required to excite a non-bonding electron. This phenomenon is called “state switching” and is a mechanism by which photoactive compounds absorb UV radiation. By enhancing the state switching in a photoprotective or sunscreen composition, a more efficient UV absorbing composition can result.
It is also known that the use of different solvents in sunscreen formulations may increase or decrease the effectiveness of a sunscreen chemical. The shifts (hypsochromic to the lower wavelength or bathochromic to higher wavelength) in the UV spectrum are due to the relative degrees of solvation by the solvent of the ground state and the excited state of the chemical.
It has been found in the prior art that as the polarity of a solvent system including a dissolved, rapidly-photodegradable compound is increased, the rate of photodecay initially decreases, but then increases again as the polarity is further increased. Thus, a photodegradable compound in solution will degrade as a second-order function of the overall polarity of the solution. Currently accepted photochemical theory provides the possibility that the mechanism by which a photodegradable compound is stabilized is the transfer of a photonically-excited electron to a nearby molecule of the same or different species (see, e.g., N. J. Turro, Modem Molecular Photochemistry, Chapter 9, Benjamin/Cummings Publ. Co., Menlo Park, Calif. (1991)), incorporated by reference herein. Additional photochemical theory is believed to coincide with the electron transfer theory of Professor Rudolph A. Marcus of the California Institute of Technology, for which he received the 1992 Nobel Prize in Chemistry, incorporated by reference herein.
U.S. Pat. Nos. 6,485,713 and 6,537,529 to Bonda et al., consistent with the above-described theory, discloses the use of amides, malates and bis-urethanes in a solvent system to control the polarity of the solvent system in a sunscreen composition. The use of these specific components results in an oil phase having a dielectric constant no greater than about 12. The named components are used in an oil-in-water sunscreen composition in an amount about 0.1% to about 40% by weight of the total weight of the composition, and more preferably about 3 wt. % to about 20 wt. %.
In addition to the above, U.S. Patent Application Publication No. 2004/0057916 A1 to Bonda et al. discloses polymers and compounds including a diphenylmethylene or a 9H-fluorene moiety for use in sunscreen compositions to photostabilize UVA sunscreen actives.
Critical wavelength is another important aspect in optimizing the performance of a photoprotective composition. In 1994, Diffey described the Critical Wavelength in vitro method, which is based on the absorption spectrum of a sunscreen product obtained via UV substrate spectrophotometry (Diffey B L (1994) A Method for Broad-Spectrum Classification of Sunscreens. Intl J Cosmet Sci, 16: 47–52), which is incorporated by reference herein. The absorption spectrum of a sunscreen is characterized by an index, namely critical wavelength, which is the wavelength where the integral of the spectral absorbance curve reached 90% of the integral from 290 nm to 400 nm. The critical wavelength method is used to determine the breadth of UV protection and is the recommended method for the evaluation of long wave efficacy of sunscreen products. Therefore, by optimizing the critical wavelength properties of a photoprotective composition, enhanced photoprotection may result.
Another measure of a sunscreen composition's efficiency is the Star Rating (UVA/UVB Ratio) according to the Boots Star Rating System (4-star that was recently revised to 5 star category). The Star Rating is calculated as an indicator of the UVA absorbance properties of a sunscreen product, relative to UVB as described in the Revised Guidelines to the practical measurement of UVA:UVB ratios according to Boots Star Rating System. The calculation of the UVA:UVB absorbance ratio will typically yield values from zero (equal to no UVA absorbance) up to 1.0 (UVA absorbance equal to UVB).
What is absent in the prior art is a photoprotective composition having one or more agents that differ from the prior art that are capable of optimizing at least one of the following properties: polarity, critical wavelength, SPF, PFA, Star Rating, or any combinations thereof, in the oil phase, water phase, both phases, or the final sunscreen formulation; thus resulting in a more efficient and photostable photoprotective composition.
The present invention addresses this shortcoming by providing an efficient photoprotective composition having one or more optimization agents capable of optimizing at least one of the following properties: polarity, critical wavelength, SPF, PFA, Star Rating, photostability or any combinations thereof, in the oil phase, water phase, both phases of the composition, or the final sunscreen formulation.